The man who is being questioned was at the home at the time.
Caboolture child protection unit detective senior sergeant Stephen Windsor would not reveal the relationship between the two.
However, family friend Tony Johns said the boy, Mason, was the son of the partner of the man being questioned.
Mr Johns said he saw the boy two days ago.
“He was a good little boy, full of life, laughing,” Mr Johns said.
“I’d walk up and he’d do like the knuckle punch with me.”
The arrested man has not been formally arrested or charged.
Detective senior sergeant Windsor did not say if police were considering it a homicide.
“We are taking one step at a time,” he said.
“The child has sustained injuries.
“It is very upsetting.”
Neighbours said there was a dispute at the house.
Neighbour Debbie Moke said police woke her at 2:00am and said there had been “a bad case of assault”.
She said the man who was being questioned by police lived at the property.
She does not know the residents, but has often seen a young boy in the yard.
“He just comes and goes, he doesn’t have much to do with the cul-de-sac,” she said.
“It’s just sad, it’s a really, really bad situation to be in.
“It’s a bit close to home.”